Multiple beam optical scanning device wherein scanning control signals are stabilized against variations in beam intensities

Abstract

A scanning device including an optical head which generates three radiation beams for scanning a track in an information plane of a record carrier. The radiation from the record carrier produced by the respective beams is detected by respective detection systems. The resulting detection signals are combined to derive scanning control signals as well as a reference signal which corresponds to variations in and differences between the beam intensities. A divider circuit uses the reference signal to stabilize the control signals against such variations and differences in the beam intensities.

I claim: 
     
       1. A device for optically scanning tracks in an information plane of a record carrier, said device comprising an optical system for generating first and second tracking beams and a main beam, a lens system for focusing the three beams on the information plane to form two tracking spots at both sides of the centerline of a track to be scanned and one main spot on said track, at least three detection systems a, b and c for respectively receiving radiation from the information plane produced by the first and second tracking beams and the main beam and deriving therefrom respective detection signals S a , S b  and S c , each detection signal being a measure of the total radiation energy incident on the relevant detection system; characterized in that said device comprises a signal processing circuit for deriving a reference signal S r  from said detection signals in accordance with   S.sub.r =S.sub.a +S.sub.b +zS.sub.c,     wherein z is a constant equal to -2Tcos (φ), φ=2πx 0  /q, x 0  is the transverse distance between a tracking spot and the main spot, q is the track pitch, and T is the intensity ratio between a tracking beam and the main beam; whereby z is zero for φ=π/2, and the reference signal S r  is a measure of the total radiation energy incident on all of the detection systems and is free of modulation by deviations between the center of the main spot and the centerline of the track being scanned.   
     
     
       2. A device as claimed in claim 1, characterized in that x 0  =1/4q, so that the reference signal S r  formed by said signal processing circuit is the sum of S a  and S b . 
     
     
       3. A device as claimed in claim 1, characterized in that x 0  =1/2q, so that the reference signal S r  formed by said signal processing circuit is the sum of S a , S b  and 2TS c . 
     
     
       4. A device as claimed in claim 1, wherein the signal processing circuit comprises a circuit for multiplying one of the signals S a , S b  by a further constant, said further constant being dependent on the ratio of the tracking beam intensities. 
     
     
       5. A device as claimed in claim 1, further comprising a normalizing circuit for normalizing a control signal derived from the detection signals and used for positioning the main spot, the normalizing circuit having an input coupled to the output of the signal processing circuit to receive the reference signal S r  produced thereby. 
     
     
       6. A device as claimed in claim 5 wherein the control signal is a tracking error signal, and further comprising a normalizing circuit for normalizing the tracking error signal in conformity with the reference signal. 
     
     
       7. A device as claimed in claim 5, wherein the control signal is a focus error signal, and further comprising a normalizing circuit for normalizing the focus error signal in conformity with the reference signal. 
     
     
       8. A device as claimed in claim 5, wherein the control signal is a track loss signal, and further comprising a normalizing circuit for normalizing the track loss signal in conformity with the reference signal. 
     
     
       9. A device as claimed in claim 5 wherein the control signal is a position signal which represents the position of an element with which the transverse position of the main spot with respect to the tracks can be adjusted, and further comprising a normalizing circuit for normalizing the position signal in conformity with the reference signal. 
     
     
       10. A device as claimed in claim 1, further comprising a flaw discriminator for detecting deviations in the reflectance of the information plane, an input of the flaw discriminator being connected to the output of the signal processing circuit to receive the reference signal S r  produced thereby. 
     
     
       11. A device as claimed in claim 10, wherein the flaw discriminator comprises a sub-circuit having an input to receive the reference signal S r  representing the radiation energy in all three beams in order to make the flaw discriminator independent of the radiation energy of said beams.